prefer_openssl: add amd64 constraint

Optimized assembly versions for Go GCM are only available
on amd64.

Documentation/MANPAGE.md

@@ -97,7 +97,7 @@ used internally for daemonization.
 
 **-openssl bool**
 :	Use OpenSSL instead of built-in Go crypto (default "auto"). Using
-built-in crypto is 4x slower unless you CPU has AES instructions and
+built-in crypto is 4x slower unless your CPU has AES instructions and
 you are using Go 1.6+. In mode "auto", gocrypts chooses the faster
 option.
 

internal/prefer_openssl/prefer_go1.5.go

@@ -1,9 +1,10 @@
-// +build !go1.6
-// = go 1.5 or lower
+// +build !go1.6 !amd64
+// not go1.6+ OR not amd64
 
 package prefer_openssl
 
 func PreferOpenSSL() bool {
-	// OpenSSL is always faster than Go GCM on old Go versions.
+	// OpenSSL is always faster than Go GCM on old Go versions or on anything
+	// other than amd64
 	return true
 }

internal/prefer_openssl/prefer_go1.6.go

@@ -1,10 +1,11 @@
-// +build go1.6
-// = go 1.6 or higher
+// +build go1.6,amd64
+// go1.6+ AND amd64
 
 package prefer_openssl
 
 // PreferOpenSSL tells us if OpenSSL is faster than Go GCM on this machine.
-// Go GCM is fastern when the CPU has AES instructions and Go is v1.6 or higher.
+// Go GCM is faster when the CPU has AES instructions and Go is v1.6 or higher
+// on amd64.
 //
 // See https://github.com/rfjakob/gocryptfs/issues/23#issuecomment-218286502
 // for benchmarks.